Currently the only possibly tax would be VAT. It would be worthwhile having proof of its VAT status (we normally think of that problem the other way round (mainland EU countries being much stricter on it than the UK). Given Dave’s comment - it would be worth keeping a paperwork trail that proves when it arrived. Ie the Bill of Sale (or receipt) and proof when it moved into the country (ferry receipt, marina/boatyard storage receipts, insurance etc).

If you are buying as a private person a new boat from a European dealer or directly from the boat manufacturer they will charge you VAT at the local rate. Once the VAT has been paid you will not have to pay VAT in the UK on arrival. You should present your boat invoice to the UK customs as proof that VAT had been paid.